LCD panels are used in various applications ranging from portable electronic devices to fixed location units, such as video cameras, automobile navigation systems, laptop PCs, and industrial machines. The LCD panel itself cannot emit light but must be back lit by a light source. The most commonly used backlight source is a cold-cathode fluorescent lamp (CCFL). However, igniting and running the CCFL requires a high AC signal. Typically, the igniting voltage is approximately 1,000 volts and the running voltage is approximately 500 volts. To generate such a high AC signal from a DC power source, e.g., a rechargeable battery, a DC/AC inverter is designed.
Besides delivering the desired high AC signal, the DC/AC inverter also faces on-going demands for ever-increasing efficiency, higher reliability, size reduction and ever-lower cost. Additionally, the DC/AC inverter is expected to deliver the AC signal with a pure sinusoidal waveform that is desirable to minimize RF emission as well as to provide optimal current-to-light conversion in the CCFL. On the contrary, a distorted sinusoidal waveform with a high crest factor will shorten the lifespan of the CCFL. The crest factor is the ratio of the peak lamp current to the average lamp current.
In terms of the operating frequency, current DC/AC inverters may be divided into two categories, the fixed operating frequency category and the variable operating frequency category. An example of DC/AC inverter with fixed operating frequency is disclosed in U.S. Pat. No. 5,619,402 and herein incorporated by reference in its entirety. This kind of DC/AC inverters has a constant operating frequency regardless of the level of the DC input signal and the load condition. Consequently, though the benefits of high efficiency, reliability, and low electromagnetic interference can be gained when the DC input signal is relatively low or the load is relatively heavy, a higher crest factor of the lamp current will be present when the DC input signal is relatively high or the load is relatively light. As a result, the higher crest factor may shorten the lifespan of the backlight lamp.
A typical example of the DC/AC inverters with variable operating frequency is a “Buck/Royer” circuit 100 as illustrated in FIG. 1. The circuit 100 is essentially a combination of a step-down buck regulator 110 and a self-resonant Royer oscillator 120 with an integral step-up transformer 121. The step-down buck regulator 110 converts an unregulated DC input signal Vdc from a battery or a potential line to a stable voltage within a rated input range of the self-resonant Royer oscillator 120. The self-resonant Royer oscillator 120 is composed of the step-up transformer 121, two power switches 123 and 125, a resonant capacitor 127, a base winding 129, a ballast capacitor 131 and a PWM controller 130. The operating frequency of the circuit 100 is set to correspond to a resonant frequency. The resonant frequency is further determined by a resonant circuit consisting of the step-up transformer 121, the resonant capacitor 127, the base winding 129, the ballast capacitor 131 as well as a CCFL load 140. Therefore, the operating frequency will change dynamically with the CCFL load condition and such an operation mechanism is called a variable operating mode.
Some derived topologies of the DC/AC inverters with the variable operating frequency can be found in U.S. Pat. Nos. 5,430,641; 5,619,402; 5,615,093; 5,818,172. Though the AC signal provided by the DC/AC inverters disclosed in these references has a good crest factor, each of them suffers from low conversion efficiency, two-stage power conversions and electromagnetic interference. Additionally, when the load is heavy or under a short circuit condition, a high magnetic flux density will be present at the transformer in the DC/AC inverter. Due to the high magnetic flux density, the transformer may be saturated and the components in the DC/AC inverter may be damaged.
Thus, there are drawbacks associated with both conventional fixed and variable frequency inverters.